1. Field of the Invention
The invention relates generally to farm guidance systems and more particularly to a farm vehicle guidance system using implement sidehill drift compensation with global navigation satellite system (GNSS) positioning.
2. Description of the Background Art
Global navigation satellite systems (GNSS), especially the global positioning system (GPS), is now commonly used for steering tractors. The tractor measures its path by processing GNSS positioning signals that it receives at a GNSS antenna. An autopilot or a driver steers the tractor by minimizing a crosstrack error between the path measured for the GNSS antenna and a desired path for a farm implement that is pulled by the tractor. Recent developments of precise positioning with differential (DGPS) and real time kinematic (RTK) GPS carrier phase corrections have made it possible for farmers to map furrows within a field and then return to those furrows with the accuracy that is required for planting and cultivating.
Two-dimensional horizontal positioning is normally used with the assumption that the tractor is on horizontal ground. The horizontal assumption enables a farm system to calibrate the position of the GNSS antenna on the tractor to a position vertically above the effective point on the ground for the implement. The GNSS antenna is most advantageously mounted high on the tractor in order to have a clear line-of-sight to the GNSS satellites. However, this presents a problem called roll error when the tractor is on a sidehill because the height of the antenna and the lateral angle of the sidehill displaces the antenna's two-dimensional horizontal position to the side of the implement ground point.
Existing farm guidance systems compensate for sidehill roll error using an inclinometer for measuring a roll angle and then using the sine of the roll angle times the height of the GNSS antenna for compensating for the roll error. However, sidehills also present tracking errors due to yaw and implement drift. Yaw error results from the lead length of the GNSS antenna in front of the effective implement ground point and the uphill crabbing angle of the tractor as the tractor attempts to compensate for its sideways downhill slippage. Implement sidehill drift is due to lateral sideways downhill slippage of the implement relative to the tractor. The effects of yaw, crabbing, and drift combine to cause an implement sidehill tracking error.
The sidehill errors of the roll error, implement sidehill drift and yaw error are all functions of the roll angle. Therefore, it might seem that a total sidehill compensation could be computed directly from the roll angle, the lead length of the antenna, and the use of a selected height that is different than the actual GNSS antenna height. The difference between the actual GNSS antenna height and the height that is used is selected in order to compensate for the combination of the effects of the roll error and the tracking errors due to yaw and implement sidehill drift.
Unfortunately, when this is done it causes a problem for the dynamics of automatic steering for the tractor autopilot. When the tractor is driving along its path on the field it is also rolling side to side due to uneven ground. This rolling has an accentuated effect on the GNSS-measured positions due to the height of the GNSS antenna. Tractor steering automatic steering systems have loop stability equations that are carefully designed with the use of the antenna height in the optimization of a tradeoff between having a fast response time and avoiding significant overshoots in the steering of the tractor. If the GNSS antenna height that used in the design of the equations is not the true height, the steering system causes the tractor path to be slow to respond or to have large side-to side oscillating errors.
There is a need for a farming guidance system using a tractor-vehicle mounted GNSS positioning system with roll and tracking compensations for sidehills.